1. Field of the Invention
This invention relates to a steering control system for a vehicle.
2. Description of the Related Art
Earlier patent applications proposed by the assignee, including those made public by Japanese Laid-Open Patent Application Nos. Hei 5(1993)-197,423 and Hei 9(1997)-221,054, teach vehicle steering control system technologies that provide steering angle assistance for keeping the vehicle on a desired course of travel along a lane of a road.
By providing fine steering assistance while preventing vehicle lane wandering, these technologies greatly reduce the burden on the driver, which tends to increase particularly during long, high-speed driving on an expressway or the like.
In view of the interference or interaction between this type of steering assistance control and driver steering, the control should preferably be effected to provide assistance through torque (steering force) rather than steering angle.
When the vehicle is equipped with both a lane holding steering torque assist control system of such type and a steering torque assist control system of the power steering, hereinafter referred to as the power steering torque assist, an issue arises regarding how to conduct communication between the two systems. Above all, there is a need to ensure high reliability of the communication between the control systems of the steering mechanism.
An object of this invention is therefore to provide a steering control system for a vehicle that, in a vehicle equipped with two types of steering torque assist control systems, ensures appropriate and highly reliable communication between the two control systems.
In order to achieve this object, there is provided a system for controlling steering of a vehicle, including steering means having an actuator which steers driven wheels of the vehicle; first steering control means for controlling the actuator; first detecting means for detecting a lane condition of a road on which the vehicle travels; second detecting means for detecting motion of the vehicle; steering assist torque calculating means for calculating a steering assist torque necessary for holding a positional relationship between the vehicle and the lane condition; torque detecting means for detecting a steering torque manually applied to the steering means by the driver; and second steering control means for calculating a torque command to be output to the first steering control means based on the steering assist torque calculated by the steering torque calculating means and the detected steering torque detected by the torque detecting means such that the torque command decreases. In the system, the second steering control means transfers to the first steering control means an index indicative of a condition under which the steering assist torque was calculated, when the second steering control means transfers the steering assist torque to the first steering control means, thereby ensuring appropriate and highly reliable communication between the two control systems.